Functional complementation between a novel mammalian polygenic transport complex and an evolutionarily ancient organic solute transporter, OSTα-OSTβ

Abstract

These studies identify an organic solute transporter (OST) that is generated when two novel gene products are co-expressed, namely human OSTα and OSTβ or mouse OSTα and OSTβ. The results also demonstrate that the mammalian proteins are functionally complemented by evolutionarily divergent Ostα-Ostβ proteins recently identified in the little skate, Raja erinacea, even though the latter exhibit only 25-41% predicted amino acid identity with the mammalian proteins. Human, mouse, and skate OSTα proteins are predicted to contain seven transmembrane helices, whereas the OSTβ sequences are predicted to have a single transmembrane helix. Human OSTα-OSTβ and mouse Ostα-Ostβ cDNAs were cloned from liver mRNA, sequenced, expressed in Xenopus laevis oocytes, and tested for their ability to functionally complement the corresponding skate proteins by measuring transport of [3H]estrone 3-sulfate. None of the proteins elicited a transport signal when expressed individually in oocytes; however, all nine OSTα-OSTβ combinations (i.e. OSTα-OSTβ pairs from human, mouse, or skate) generated robust estrone 3-sulfate transport activity. Transport was sodium-independent, saturable, and inhibited by other steroids and anionic drugs. Human and mouse OSTα-OSTβ also were able to mediate transport of taurocholate, digoxin, and prostaglandin E2 but not of estradiol 17β-D-glucuronide or p-aminohippurate. OSTα and OSTβ were able to reach the oocyte plasma membrane when expressed either individually or in pairs, indicating that co-expression is not required for proper membrane targeting. Interestingly, OSTα and OSTβ mRNAs were highly expressed and widely distributed in human tissues, with the highest levels occurring in the testis, colon, liver, small intestine, kidney, ovary, and adrenal gland.